The present invention is generally related to hitch mechanisms for tractors and more particularly to the structure of a coupling hook which is the upper link of a three-point linkage system for a tractor hitch.
A hook of the type to which the present invention relates normally includes a hook body defining a concave recess adapted to receive in locking engagement therein a catch ball or a coupling sphere connected with the coupling pin of an implement which is to be connected or hitched to the tractor. Such devices usually include a spring loaded locking bolt which protrudes through a rectilinear opening into the recess of the coupling hook with a curved surface adapted to engage the coupling sphere. The locking bolt may be pivotally connected with a control lever at its end remote from the end extending into the hook recess with stop means being provided to limit the movement of the locking bolt in the disengaging direction of the hook structure.
Coupling hooks for receiving coupling spheres attached upon a coupling pin of a hitch system for an implement to be mounted upon a tractor are generally known. In such devices the spring-loaded locking bolt protruding with its curved locking surface into the hook recess is adapted to the radius of the ball or coupling sphere, as will be known from prior German Pat. No. 1 457 705. In such prior art devices, the object is to facilitate coupling and uncoupling of an upper coupling pin at the implement end of the tractor from the driver's seat so that coupling and uncoupling of an implement may be provided in a simplified operation. In such a device, the tension medium for the closing member of the hook and the tension medium for the catch for the longitudinal adjustment of the upper link are joined with each other.
In a further development of devices in accordance with the invention, the catch is provided with a lever designed together with the lever of the closing member of the hook as a two-arm lever where the fulcrums of the two levers are arranged close together and where the free ends of the levers are joined with each other by a traction-transmitting member.
A disadvantage of prior art devices is that a single control element is provided for the common control mechanism in order to secure the longitudinal adjustment of the upper link and for unlocking of the coupling hook, and as a result maneuverability of the upper link is impaired. In many cases it becomes necessary to change the length of the upper link with the coupling hook locked. This can be desirable, for example, in order to vary the working depth of an attached implement such as a plow.
However, in a connection of the two locking means for the longitudinal adjustment of the upper guide rod and for the coupling hook, there is a risk that during adjustment of the length of the upper link the coupling hook will become unlocked and the mounted implement will drop out of the upper hook or link. This would cause, apart from an increased risk of accident, the necessity for once again attaching the entire upper coupling point.
A further disadvantage of previously known devices involving coupling hooks of the type discussed is that the lock of the catch ball in the coupling mouth can open by rotation of the catch ball caused by tilting movements of the mounted implement due to frictional forces acting on the locking bolt.
Further suggestions of the prior art provide for a coupling hook to prevent accidental unlocking by tilting movements of the mounted implement, as will be seen from German Pat. No. 1 937 764. The coupling hook underlying this device is equipped with a safety bolt wherein there is provided a longitudinal bore, with a safety pin being loaded with a compression spring arranged within the longitudinal bore and with a device locking the axial displacement acting on the safety bolt in a locking position so that the locking device is unlocked when the safety pin is withdrawn.
Locking and unlocking of a safety bolt is effected by one or more locking balls which block the safety bolt against axial displacement relative to the housing of the coupling hook when in the locked position, and also relative to the safety pin in the unlocked position.
The coupling hook mechanism represented by the foregoing usually has a form-locking safety feature against accidental opening of the coupling hook but it is generally extremely elaborate due to the great number of moving parts which must move relative to each other and which are held within narrowly fitting elements.
In addition to the foregoing, coupling hooks of the prior art may be highly susceptible to functional difficulties because the elaborate mechanism will have its freedom of movement impaired by dirt or deleterious objects entering the parts of the mechanism.
The present invention is directed toward providing a coupling hook structure for the upper link of a tractor coupling system which receives the catch ball or coupling sphere of the implement to be coupled with the tractor and which is provided with substantial safety features against accidental unlocking, which consists of relatively few moving parts and which is generally insensitive to penetration by dirt or other contaminants.